A curing system employing energy beams such as ultraviolet rays has become an important method for improving productivity and solving recent environmental problems. The mainstream of current photo-curing systems is a free radical curing system employing (meth)acrylate materials. However, materials of a cationic curing system employing a material such as epoxy, vinyl ether and oxetane have an advantage over a free radical curing system in that (a) they are excellent in surface and thin film curability since they are hardly susceptible to cure inhibition by oxygen; (b) they have small curing shrinkage and have good adhesion to a wide range of substrates; and (c) the amount of residual monomers can be suppressed to a low level because active species have a long life and the curing gradually proceeds even after the completion of light irradiation (dark reaction). Therefore, in recent years, the materials of a cationic curing system are investigated for applications in coatings, adhesives, sealers for displays, printing inks, three-dimensional shaping, silicone-based release paper, photoresists, sealers for electronic components and the like (refer, for example, to Non-patent Document 1).
The compounds mainly used in the photo-cationic curing system include epoxy compounds, and among others, alicyclic epoxy compounds rich in reactivity are heavily used (refer to Patent Document 1). These alicyclic epoxy compounds have better surface curability than compounds of a free-radical system in that they are not susceptible to cure inhibition by oxygen even in the air. However, they have a problem that the rate of reaction is reduced as polymerization proceeds, resulting in insufficient internal curability and leading to insufficient physical properties of the cured products. Moreover, there are only a small number of types in the compounds that can be used, and it is difficult to control physical properties of the cured products to be obtained. On the other hand, there are abundant types of glycidyl type epoxy compounds widely used in heat curing as compared with the above-described alicyclic epoxy compounds. However, the glycidyl type epoxy compounds are insufficient in reactivity when they are subjected to photo-cationic curing. Therefore, some technique must be used such as use of special initiators such as SbF6 and AsF6 which have problems in safety or use of heat curing in combination. In relation to the above, oxetane compounds are known to enhance the rate of polymerization when they are used in combination with epoxy compounds since the oxetane compounds are slow in initiation reaction but high in the rate of propagation reaction when they are used alone in photo-cationic polymerization. However, also in this case, photo-curability is not sufficient yet as compared with the conventional free-radical system, and it is necessary to further increase sensitivity. Other than the above, it is known that vinyl ether compounds have high cationic polymerizability, but there is a problem that cured films obtained by homopolymerization of the same have larger shrinkage on curing than those of the above-described epoxy or oxetane, leading to reduction of adhesion with a substrate.
Thus, a number of attempts have been made to increase polymerizability of epoxy compounds and maintain adhesion by using vinyl ether compounds in combination with the above-described alicyclic epoxy compounds (refer, for example, to Patent Documents 2 and 3). Moreover, attempts have been made to use the compositions obtained from this combination in specific applications, for example, as compositions for stereolithography (Patent Document 4), overcoat agents for optical disks (Patent Document 5) and ultraviolet ray curable compositions for ink jet recording (Patent Document 6). However, in the compositions in which vinyl ether compounds are simply combined with epoxy compounds in the manner as described above, the reactivity of the epoxy compounds is indeed a little improved, but the reactivity of the vinyl ether compounds is greatly reduced, resulting in the fact that sufficient physical properties of cured films cannot be obtained. In the above-described attempts (Patent Documents 4 and 5) for the purpose of improving the above problem, polyfunctional vinyl ether compounds have been employed, but the effect of these compounds is not yet sufficient.
On the other hand, attempts have been made to enhance the curability by using hydroxyl group-containing compounds such as aliphatic polyhydric alcohols (Patent Documents 7 and 8) or phenol compounds (Patent Documents 9 and 10) as optional components in the cationic composition composed of epoxy compounds. However, in the case of the aliphatic polyhydric alcohols, there is a problem that although reactivity of the epoxy compounds is improved, the cured films are weakened. In the case of the phenol compounds, there are disclosed phenol compounds having one or two phenolic aromatic rings such as dihydroxybenzene and trihydroxybenzene and resole type phenol compounds having a methylol group in a molecule. However, all of them have insufficient curability, and in particular when resole type phenol compounds are used, there is a problem that hardness of the cured films is gradually reduced.
In addition, a technology to add phenol compounds is disclosed also in Patent Document 11. In this case, it is possible to store the cationic composition containing an unstable initiator for a long period of time because the phenol compounds act as a radical inhibitor or an antioxidant. However, it is impossible to expect the improvement of the photo-curability itself of the composition.
[Patent Document 1] U.S. Pat. No. 3,794,576
[Patent Document 2] Japanese Patent Laid-Open No. 06-298911
[Patent Document 3] Japanese Patent Laid-Open No. 09-328634
[Patent Document 4] Japanese Patent No. 2667934
[Patent Document 5] Japanese Patent Laid-Open No. 04-120182
[Patent Document 6] Japanese Patent Laid-Open No. 09-183928
[Patent Document 7] Japanese Patent No. 1266325
[Patent Document 8] National Publication of International Patent Application No. 2001-527143
[Patent Document 9] National Publication of International Patent Application No. 2002-509982
[Patent Document 10] Japanese Patent No. 3251188
[Patent Document 11] Japanese Patent Laid-Open No. 2002-69269
[Non-Patent Document 1] Masahiro Tsunooka, et al., “Industrial Development of Cationic Curing Technology”, MATERIAL STAGE, published by Technical Information Association, Vol. 2, No. 2, p. 39-92, May 10, 2002